Sintered magnetic alloy and methods of production



June 20, 1961 E. ADAMS ET AL 2,988,806

SINTERED MAGNETIC ALLOY AND METHODS OF PRODUCTION Filed Nov. 1'7, 1958 H(OERSTED) B(Ku oeAuss) l I l I l l I H(OERSTED) INVENTORS. EDMOND ADAMS WILLIAM M. HUBBARD United States Patent 2,988,806 "SlNI'ERED MAGNETIC ALLOY METHODS OF PRODUCTION Edmond Adams and William M. Hubbard, Silver Spring,

Md., assignors to the United States of America as represented by the Secretary of the Navy Filed Nov. 17, 1958, Ser. No. 774,562 16 Claims. (Cl. 29-182.5) (Granted under Title 35, U8. Code (1952), see. 266) The invention described herein may be manufactured "good magnetic properties. This alloy, well known in the art as Sendust, is described in U.S. Patent 2,193,768 to Masumoto et a1. Sendust alloy is too brittle and hard to be machined, so that it must be cast by conventional foundry techniques to the shape desired. Since thin sections or close tolerance pieces cannot be made by the usual casting procedure without cracking and Warping, it is unsuitable for use in A.C. power applications because of the high eddy current losses in the unlaminated cast material when the frequency is above a few cycles per second.

Furthermore, it is di flicult to sinter a powdered Sendust alloy because extreme compacting pressures are necessary to produce a sintered compact displaying desirable physical and magnetic properties.

It is an object of this invention, therefore, to provide a sintered alloy of the Sendust type which may be readily formed into thin sheets and complex shapes.

Another object is the provision of a process for producing a new and improved alloy containing no critical or strategic material which alloy is adapted for use in A.C. power' electromagnetic applications.

Still another object of the invention is to provide a sintered iron-aluminum-silicon alloy having magnetic properties which are not affected greatly by small amounts of impurities in the sintered compact or by slight variations in composition.

A further object is to provide a method of sintering an iron-aluminum-silicon alloy to produce a material having desirable A.C. magnetic properties.

These and'many other objects will become more readily apparent when the following specification is read and considered along with the accompanying drawing wherein:

FIG. 1 is a graph of the 60 cycle hysteresis loop of a typical cast Sendust alloy; and I FIG. 2 is a graph of the 60 cycle hysteresis loop of a typical laminated, sintered alloy fabricated according to the teachings of this invention.

The composition of the alloys used in practicing this invention lie in the range 6-12% silicon, 4-9% aluminum and the remainder iron. Optionally, small amounts of well known additives such as antimony, beryllium, nickel, chromium, tungsten, molybdenum, magnesium, manganese, vanadium, tantalum, titanium, tin, zinc, boron, copper, phosphorus, arsenic, sulfur or zirconium may be added in accordance with conventional practice to modify the physical properties of the alloy, such as resistivity,

"hardness, permeability or coercive force.

The iron, aluminum and silicon used in practicing this invention may be obtained from elemental iron powder plus alloy powders containing sufficient iron, silicon and aluminum to yield the desired final composition of the sintered product. The mixture of powders is then consolidated into an essentially solid mass by heat treatment in an inert atmosphere or vacuum accompanied optionally by compaction under pressure.

Example I A test magnetic toroid containing iron, 9.6% silicon and 5.4% aluminum was prepared from iron powder, an alloy powder containing 15% silicon-85% iron, and an alloy powder of 50% aluminum-50% silicon. The powders were mixed in correct proportions to yield the desired final composition and were then placed in a die and compacted at a pressure of approximately 40 tons p.s.i. The compact was heat treated in a helium atmosphere at 1175 C. for about 8 hours and slowly cooled. The sintered piece was essentially a solid body exhibiting a density equal to 85 to of that of the cast alloy and having initial and maximum permeability comparable to those of the cast alloy (see typical values Table 2, row 1).

Example II properties tabulated in row 2 and required extremely high compacting pressures to produce even a low density sintered compact.

Powders of FeSi alloy, AlSi alloy and Fe produce a sintered compact superior to that obtained by using powders of elemental Fe, Al and Si. Although the reason for this is not entirely clear, it appears to be that elemental aluminum, when heated and fused, forms an oxide coating on its surface. Even if the heating process is carried out in an inert atmosphere, there is sufiicient residual oxygen in the green compact to form this oxide coating which prevents the aluminum from wetting the solid iron and silicon. Consequently, liquid phase sintering does not take place and the rate of diffusion of the powdered components is slowed thereby necessitating longer heating times and higher compacting pressures. On the other hand, when aluminum is present in the form of an AlSi alloy, the reaction is not inhibited by the formation of oxide on the surface of the molten AlSi alloy thus the liquid phase Wets the iron rich powder thereby permitting diffusion of the solid particles throughout the liquid phase. This results in a high density sintered compact which is more homogeneous and requires less severe sinteri-ng temperatures and compacting pressures.

The composition of the AlSi alloy could vary from 20% Al 80% Si to about 89% Al 11% Si depending on the desired final composition of the sintered compact. The melting point of the 20% Al alloy is 1300 C. which is about the highest practical sintering temperature and the melting point of the 89% Al alloy is 577 C. This latter temperature is too low because it is usually necessary to sinter at about l000 C. to 1300 C. to achieve a fast reaction rate. A 55% Al 45% Si alloy melts at 1000" C. and forms a satisfactory liquid phase for sintering at this temperature; this alloy contains about the highest percentage of aluminum that is practical for practicing this invention.

Although silicon, aluminum and iron may be added as elemental particles, it is preferable to add any silicon required in excess of the amount provided by the AlSi alloy as FeSi alloy. Since it is necessary for the Fe and Si to diifuse and intimately commingle, alloying the additional Fe and Si necessarily hastens the diffusion process.

It has been determined that adding the Al and Si as .FeAl and JFeSi produces a poor grade compact which requires extremely high compacting pressures (see Table 2 rows 4 and 5).

Additional aluminum may be added as elemental powder without detrimental effects provided that sufficient AlSi alloy is present to form the liquid phase. Elemental iron may be employed in order to attain the desired final composition. Example "I The process of Example I was repeated except that instead of a helium atmosphere the compact was heated in a acuum. The toroid tested exhibited magnetic properties significantly better than those of the toroid in Example II but inferior to the properties of the toroid of xamp I ee Table )-v Example IV Powders of Fe, FeSi and FeAl were mixed in correct proportion to produce a mixture containing 85 iron,

9.6% silicon and 5.4% aluminum. The mixture was processed as in Example I. The compact produced displayed rather poor magnetic properties as indicated in Table 2. Example V The powders of Example IV were sintered by heating in hydrogen instead of helium. It was necessary to increase the sintering temperature to 1300 C. in order to form a cohesive compact, even at that high sintering temperature, the material displayed even less desirable mag netic properties than Example IV.

Example VI Example VII A toroid containing 85% iron, 9.6% silicon and 5.4% aluminum was made by powdering a cast Sendust alloy of appropriate composition, then placing the powder into a die and compacting it at a pressure of 40 t.s.i. (tons per square inch). The resulting compact displayed very low green strength so that extreme care was necessary in transferring it to the sintering furnace. The green compact was heated to 1175 C. in a vacuum for about 8 hours then slowly cooled. The toroid thus formed exhibited magnetic properties which were slightly better than those obtained by the process of Example I. However, the use of ground Sendust is less desirable than powders of silicon-aluminum alloys mixed with powders of elemental iron because the compacted green Sendust core is weak and excessively friable.

In the foregoing examples, the composition of the sintered compact was maintained at 5.4% aluminum, 9.6% silicon and 85% iron since that composition optimizes the magnetic properties of the material. The range of permissible compositions is from 4 to 9% aluminum, 6-12% silicon, the balance essentially iron.

Table I indicates the effects on the magnetic properties obtained by varying the relative proportion of the aluminum, silicon and iron in the sintered compound:

All of the compositions in the range 612% silicon, 49% aluminum have usable magnetic properties. While the selection of a particular composition may depend largely upon the use to which the material will be put, 5.4% aluminum, 9.6% silicon and iron is the most desirable proportion for the greater number of applications.

In the cast alloy, the magnetic properties change abruptly if the composition of the alloy is slightly varied. For instance, the Masumoto et al. patent indicates that as the composition of the alloy is changed from 84.13% iron, 6.21% aluminum, 9.6% silicon to 88.35% iron, 6% aluminum, 10.5% silicon the maximum permeability (u falls from 162,000 to 20,000, an eight fold decrease. The sintered alloy, on the other hand, is rather insensitive to small variations in the alloy composition and to minute impurities. This is probably due to the fact that it is not completely homogeneous because of voids and other phase discontinuities throughout the sintered compact. This inhomogeneity tends to make the compact much less sensitive to slight fluctuations in composition which invariably occur in an industrial process.

As stated hereinbefore, the sintering takes place in a complex AlSi liquid phase. When the pressed compact is heated, an aluminum-silicon alloy melt is formed, which wets the iron and FeSi alloy in the green compact so that virtually no pressure is required to form the sintered compact. One advantage which inures from this liquid phase sintering is that very thin sheets of the sintered alloy can be fabricated. The sheets can then be used to form magnetic articles possessing low hysteresis losses. This phenomena also makes it possible to slip cast the material since in slip casting very little pressure is put upon the material.

Example VIII Powders of FeSi alloy, AlSi alloy and Fe are mixed in correct proportions to give a ratio of 85 iron, 9.6% silicon and 5.4% aluminum. These powders are suspended in a water slurry which optionally has a small amount of polyvinyl alcohol and/or ammonium alginate to give a fieely flowing suspension which is poured onto a flat surface to give a film of uniform thickness from about .004 inch to any desired thickness. The film is then dried and allowed to set up. A green material of the desired shape is cut out with a cookie cutter type tool and heat treated in helium at 1150 C. for 8 hours to sinter the mixture and provide a structure having the desired physical and magnetic properties. Intimate contact of the powder particles is not required to provide a dense sintered compact because the alloy is sintered in the AlSi liquid phase which wets the iron so that difiusion throughout the mixture takes place very rapidly without compaction. Optionally, the mixture may be partially heat treated before the final shape is cut to produce a stronger green compact from which to cut the final shape thereby reducing breakage somewhat.

The time and temperature employed in any of the above preferred examples are interrelated, for example the time and temperature might vary from 16 hours at 1000 C. to about 1 hour at 1250 C. However, 1000 C. is about the lowest practical temperature at which to form the complex AlSi liquid phase. The optimum temperature for sintering is about 1175 C.; although higher temperature may be employed, it appears that they produce slightly inferior magnetic properties. If continuous sheets are desired (greater than 0.010") the FeSi, AlSi, Fe powder may be rolled into green sheets and sintered according to the above described process.

A toroid made of laminations 0.0225 inch thick was tested and exhibited A.C. magnetic properties shown by the 60 cycle hysteresis loop of FIG. 2. As is apparent from FIG. 1, the cast alloy .188 inch thick exhibits very poor A.C. properties as compared with those of the laminated sintered alloy having laminations .0225 inch thick.

con, 5.4 aluminum, 85 percent iron:

FTABLEH Obviously many modifications and variations of the present invention are possible in the light of the above teachings. -It is therefore to be understood that the scope of the appended claims the invention may be practiced otherwise than as specifically described. a

What is claimed as new and desired to be secured by Letters Patent of the United States is:

l. The process of forming a sintered compact con No. Powders Employed Atmosphere, #20 l me: Burn: BRosidual He Lowest Comp ction T, 0. Pressure Required Fe, rest, AlSi; 7, s35 0.20 No pressure.

' ,1 170 5, 440 0. 22 100 t.s.i.

. 81 3, 770 0. 19' No pressure. Fe, FeAl, FeSi- He l,175 2,105 6,000 6, 210 2, 440 0. 24 40 .s.i.

Fe, FeAl, FeSi H2 l,300 2, 500 5,000 6, 466 1, 210 0.22 100 t.s.i. 90gblssie)ndust Plus 10% (Fe, FeSi, He 1,175 8, 000 28, 980 10, 980 4,087 0. 12 40 t.s.i. Powdered Sendust He 1,175... 6, 667 23 350 10, 880 4, 563 0.08 Pigrt Gireen Strength,

.5. 90% Flaked Sendust Plus 10% He 1,175 10,000 29 144 11, 725 4, 759 0.12 40 t.s.i.

(Fe, FeSi, AlSi). Sendust Alloy (as cast) None. 9,050 43 130 10,200 3, 040 0.07

Referring to this table, it is apparent that the elementary powders (row 2) are entirely unsatisfactory because of the extreme compaction pressures required. Similarly, heating in a hydrogen atmosphere (row results in a compact of poor magnetic properties and requires a sintering temperature of 1300 C. which is at least 100 C. higher than required using the other techniques. Although powdered Sendust (row 7) exhibits good magnetic properties, it is weak and friable before heat treatment and consequently must be handled with special care. Because of the hardness of Sendust alloy, compaction pressures of at least 40 tons p.s.i are required to develop its magnetic properties, therefore, the Sendust powder cannot be mixed into a slurry and slip cast or flowed into thin sheets in the manner of Example VIII.

Addition of percent by weight of powdered AlSi alloy to the Sendust powder (row 6) or flake (row 8) increases the green strength of a Sendust compact sufliciently to make it easy to handle; at the same time the magnetic properties remain good. However, compaction pressure of 40 tons is required, making it impossible to flow a slurry of this powder into a thin sheet. In the event, however, that it is desirable to compact a powder into a complex shape rather than to form a thin sheet, the techniques of numbers (6) and (8) in Table 2 may be employed. The DC. magnetic properties of sintered, powdered FeSi, AlSi and Fe develop without any com paction pressure. Therefore, these powders are ideally suited for use in fabricating a thin sheet of sintered material.

The preferred powder compositions are the FeSi, AlSi, Fe powders and powdered or flaked Sendust with at least 10% FeSi, AlSi and Fe powders. The selection of FeSi, AlSi, Fe or the Sendust plus FeSi, AlSi and Fe powders depends upon the mode of forming the finish piece i.e. for slip casting FeSi, AlSi, Fe powders must be used while if the piece is compacted under pressure, powders of Sendust plus the FeSi, AlSi, Fe may be used. Increasing the amount of powder FeSi, AlSi, Fe and decreasing the Sendust flake or powder tends to degrade the magnetic properties of the alloy slightly but also decreases the compaction pressure required. From the experimental results set out hereinbefore it is evident that the sintered alloys of this invention and the method of their fabrication may be used to produce extremely versatile magnetic components which exhibit good electromagnetic properties, require no strategic material such as niokel or cobalt, and are easily workable into a desired configuration.

taining about 49% total Al, 612% total Si, the remainder essentially Fe, which comprises the step of mixing together powders of aluminum, iron and silicon, said aluminum being present entirely in the form of an aluminum silicon alloy and heating the mixed powders to a temperature of at least 1000 C. in an inert atmosphere to sinter the powders into a solid compact having desirable magnetic properties, the sintering occurring in a complex AlSi liquid phase to inhibit oxidation of the aluminum during sintering.

2. The process of claim 1 wherein said powders are heated in a vacuum.

3. The process of claim 1 wherein said powders are heated in an atmosphere consisting essentially of at least one of the noble gases.

4. The process of claim 3 wherein the noble gas is helium.

5. The process of forming a sintered material comprising the steps of; powdering a cast alloy containing 49% Al, 612% Si the remainder essentially Fe, mixing the resultant powder together with powders of AlSi alloy, FeSi alloy and Fe said second powders containing about 49% Al, 6-12% Si, the remainder essentially Fe, so that the second powders form at least 10% of the total mixture, compacting the mixed powders, heating the compacted powders to a temperature between 1000 C.l300 C. in an inert atmosphere for 1 to 16 hours to sinter the powders into a solid compact having desirable magnetic properties, the sintering occurring in a complex of AlSi liquid phase to inhibit oxidation of the aluminum during sintering.

6. The process of claim 5 wherein powder mixture contains 5.4% A1, 9.6% Si and Fe.

7. The process of claim 5 wherein the powders are sintered at a temperature of 1175 C. for about eight hours.

8. The process of claim 7 whereon the powders are sintered in a helium atmosphere.

9. The process of claim 5 wherein the powders are sintered in helium.

10. The process of forming a thin sheet of a sintered magnetic material comprising the steps of mixing a slurry of a carrier liquid with powders of AlSi alloy, FeSi alloy and Fe containing on a dry weight basis about 4-9% Al, 6-12% Si, the remainder essentially Fe, forming a thin layer of the slurry, evaporating the carrier liquid to form an unsintered sheet, heating said sheet at 1000" C.-1300 C. for l-16 hours in an inert atmosphere 7 to sinter the sheet in a liquid phase thereby forming a dense electromagnetic material. a

' 11; The process of claim 10 wherein said sheet is heated for 8 hours at 1175C.

12. The process of claim 11 wherein said sheet is heated in a helium atmosphere.

13. The process of claim 12 wherein said powders contain about 5.4% A1, 9.6% Si and 85% Fe.

14. The process of forming a sintered electromagnetic sheet comprising the steps of; forming a slurry of powders of AlSi alloy onta n ng 2971-5571 Al, h rem inder essentially Si, FeSi alloy and Fe, said powders containing on a dry weight basis about 4-9% total A1, 6-12% total Si, the balance essentially Fe, slip. casting the slurry to form a shaped unsintered material, heating said material in an inert atmosphere at 117-5" C. for a time sufficient 10 during sintering.

References Cited in the file of this patent UNITED STATES PATENTS Howe Mar. 5,1949 

1. THE PROCESS OF FORMING A SINTERED COMPACT CONTAINING ABOUT 4-9% TOTAL AL, 6-12% TOTAL SI, THE REMAINDER ESSENTIALLY FE, WHICH COMPRISES THE STEP OF MIXING TOGETHER POWDERS OF ALUMINUM, IRON AND SILICON, SAID ALUMINUM BEING PRESENT ENTIRELY IN THE FORM OF AN ALUMINUM SILICON ALLOY AND HEATING THE MIXED POWDERS TO A TEMPERATURE OF AT LEAST 1000*C. IN AN INERT ATMOSPHERE TO SINTER THE POWDERS INTO A SOLID COMPACT HAVING DESIRABLE MAGNETIC PROPERTIES, THE SINTERING OCCURING IN A COMPLEX ALSI LIQUID PHASE TO INHIBIT OXIDATION OF THE ALUMINUM DURING SINTERING. 